Part of the issue is that terrestrial primates aren't comparatively strong - size is useful, for both mating and defensive displays, so we've leaned toward size without the metabolic requirements that size might require of lean and efficient muscles. If you're arboreal, maximum power with minimum weight is better.

And for little animals, it's a combination of smaller muscles being relatively stronger, and a lot of animals whose strength you hear about (like ants) have exoskeletons, which are more kinetically powerful than endoskeletons. More power translated to movement, less lost in connections.

I listen that gorila's arm is ~20 (!!!) times more stronger that humans average arms, ind chimpanzee is more stronge about 4 times... By wigght comparison - gorilla is propotionally stronger that chimpanzee, and humans is weekest hominids. Also, esplanation can be in the ligaments structure, that thin at the humanns - for more exactly movements...

A shift in development towards a larger cranium (as opposed to muscular strength) was also one of the hallmarks of human evolution. As they relied more on their brains and social behavior, they had to rely less on physical strength to survive and propagate.